The present invention relates to a multi-color image forming apparatus based on the xerography process, such as a copying machine, a facsimile machine, or a printer. More particularly, the invention relates to a drive device for a rotary type developing unit which has a plural number of developing subunits arrayed around a rotary body, and is capable of developing toner images of at least two colors.
In a color image forming process by a multi-color image forming apparatus based on the xerography process, a photoreceptor is uniformly charged. Latent electrostatic images of four colors, black, yellow, magenta, and cyan are formed on the surface of the charged photoreceptor in successive order. These electrostatic images are developed into toner images of those colors by the developing subunits for these colors, which are arrayed around a rotary body. The developed toner images are transferred onto a paper being transported by a transfer drum, which is being turned in proximity with an image bearing means. Through four turns of the transfer drum, the toner images of four colors are superposedly transferred onto the paper. The resultant image is a color image of four colors. (For this multi-color image forming apparatus, reference is made to Published Unexamined Japanese Patent Application No. Sho. 63-128375.)
The developing subunit includes a developing roll disposed in proximity with the photoreceptor, a magnet roll for forming a plurality of magnetic patterns, and a developing sleeve rotatably supported around the magnet roll. Developer is attracted to the surface of the developing sleeve by a magnetic force developed by the magnet roll. As a result, magnetic carriers are raised to form a magnetic brush. With rotation of the developing sleeve, the magnetic brush on the surface thereof is transported to a developing region where the developing sleeve faces the photoreceptor. In this region, the magnetic brush is made close to or to contact with the photoreceptor. At this time, a bias voltage is applied between the developing roll and the photoreceptor. A potential acting such that the charged toner is attracted to the photoreceptor, is set up between an image forming area on the photoreceptor and the developing roll. A potential acting such that toner is attracted to the surface of the developing roll, is set up between a nonimage forming area and the developing roll. In this state, the charged toner is transferred to only the image forming area on the photoreceptor, to thereby visualize the latent electrostatic image.
FIGS. 12 to 14 cooperate to show a conventional rotary developing unit. Of those figures, FIG. 12 is a sectional view of the developing unit when seen from one side. FIG. 13 is a side view, partly in cross section, showing a drive system for the developing unit. FIG. 14 is a timing diagram showing the operation of the developing unit. In FIGS. 12 and 13, the rotary developing unit includes four developing subunits 2a to 2d, which are equidistantly arrayed along the circumference of a rotary body 7. The rotary body 7 is rotatably supported by a rotary shaft 7a, which is supported on a main frame of the rotary developing unit 1. The rotary body 7 thus supported may be turned by a drive motor, not shown, in the direction of an arrow in the drawing. A photoreceptor drum 8 is disposed in proximity with and faces the rotary body 7, and is turned by a drive motor, not shown, in the direction of an arrow shown in the figure.
A clutch 12 for driving the developing unit is mounted on a bracket 11 of an apparatus body frame 10. A power is transmitted from the photoreceptor drive motor to the clutch 12, through a belt 13. A drive shaft 15, supported on the body frame 10 through a bearing 14 placed therebetween, is inserted into the clutch 12. The power is transmitted to the drive shaft 15 through the on/off operation of the clutch 12. A gear G0 for driving the developing subunit is fastened onto the end of the drive shaft 15. A follower gear G3 is in mesh with the gear G0. The follower gear G3 is supported by a support shaft 18 through a bearing 21 placed therebetween. The support shaft 18 is fastened to a bracket 25 that is mounted on the rotary body 7. A rotation of the follower gear G3 is transferred to a follower gear G4, a developing roll drive gear G5, a follower gear G6, a developer agitating/transporting auger drive gear G7, a follower gear G8, and a developer agitating/transporting auger G9, in this order. With the rotation of these gears, a developing roll, not shown, is turned.
The operation of the rotary developing unit thus constructed will be described with reference to FIG. 14.
(A) The drive motor for the rotary body 7 is turned on ((1) in FIG. 14). PA1 (B) The rotary body 7 moves a first developing subunit to its developing position, and stops it thereat. After completion of the developing operation by the first developing subunit, the rotary body 7 moves a second developing subunit to its developing position, and stops it thereat ((2) in FIG. 14). PA1 (C) This operation is repeated for the remaining third and fourth developing subunits, thereby developing a full color image. PA1 (D) The clutch 12 is put in a coupling state at a time point y1 after the developing subunit reaches its developing position, and is turned off at a time point x1 just before the developing subunit leaves the developing position. The same thing is correspondingly applied to the application and removal of the bias voltage ((3) and (4) in FIG. 14).
The rotary developing unit is constructed such that no drive force is applied to the developing roll as a developer bearing means only when the developing subunit is moved on its circular path and reaches its developing position. Therefore, such a state that the developer bearing means rolls to a standstill at the developing position, and faces the photoreceptor while being in proximity with the latter, is set up in the rotary developing unit. In this state, developer at the developing nip of the developer bearing means where it faces the photoreceptor is stuck onto the surface of the developer bearing means. The developer stuck thereonto appears as streaks in the reproduced image.
Moreover, in the conventional control method for controlling the application of a bias voltage, a preset rotation time of the rotary body that is previously computed is frequently incoincident with an actual rotation time thereof. This results in degradation of image quality, e.g., fog appearing on the reproduced image and carrier being stuck onto the image bearing means.
The image forming speed of the recent image forming apparatus is increased. With increase of the image forming speed, a rotation speed of the image bearing means becomes higher, so that a high speed movement of the developing unit is required. In this circumstance, the application of the bias voltage to the developing subunits must be more accurately timed. Where an image forming area on the image bearing means is limited in accordance with the size of a paper used, use of the two-component developer arises a problem when the developing subunit is placed to face the nonimage forming area of the image bearing means. That is, in such a situation, carriers of the developing subunit are scattered onto the nonimage forming area, to possibly damage the surface of the image bearing means. To cope with this problem, there is a proposal that the rotation speed of the rotary body is controlled in accordance with the size of the paper so as to prevent each developing subunit from facing the nonimage forming area of the image bearing means. The proposal makes the control of the bias voltage application more intricate.